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OpenSSL

Note:
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OpenSSL is probably the most well known cryptographic library, used by thousands of projects and applications.

The OpenSSL configuration file is located at /etc/ssl/openssl.cnf and is used both by the library itself and the command-line tools included in the package. It is simple in structure, but quite complex in the details, and it won’t be fully covered here. In particular, we will only cover the settings that control which cryptographic algorithms will be allowed by default.

Structure of the config file

The OpenSSL configuration file is very similar to a standard INI file. It starts with a nameless default section, not inside any [section] block, and after that we have the traditional [section-name] followed by the key = value lines. The SSL config manpage has all the details.

This is what it looks like:

openssl_conf = <name-of-conf-section>

[name-of-conf-section]
ssl_conf = <name-of-ssl-section>

[name-of-ssl-section]
server = <name of section>
client = <name of section>
system_default = <name of section>

See how it’s like a chain, where a key (openssl_conf) points at the name of a section, and that section has a key that points to another section, and so on.

To adjust the algorithms and ciphers used in a SSL/TLS connection, we are interested in the “SSL Configuration” section of the library, where we can define the behavior of server, client, and the library defaults.

For example, in an Ubuntu Jammy installation, we have (omitting unrelated entries for brevity):

openssl_conf = openssl_init

[openssl_init]
ssl_conf = ssl_sect

[ssl_sect]
system_default = system_default_sect

[system_default_sect]
CipherString = DEFAULT:@SECLEVEL=2

This gives us our first information about the default set of ciphers and algorithms used by OpenSSL in an Ubuntu installation: DEFAULT:@SECLEVEL=2. What that means is detailed inside the SSL_CTX_set_security_level(3) manpage.

Note:
In Ubuntu Jammy, TLS versions below 1.2 are disabled in OpenSSL’s SECLEVEL=2 due to this patch.

That default is also set at package building time, and in the case of Ubuntu, it’s set to SECLEVEL=2.

The list of allowed ciphers in a security level can be obtained with the openssl ciphers command (output truncated for brevity):

$ openssl ciphers -s -v DEFAULT:@SECLEVEL=2
TLS_AES_256_GCM_SHA384         TLSv1.3 Kx=any      Au=any   Enc=AESGCM(256)            Mac=AEAD
TLS_CHACHA20_POLY1305_SHA256   TLSv1.3 Kx=any      Au=any   Enc=CHACHA20/POLY1305(256) Mac=AEAD
TLS_AES_128_GCM_SHA256         TLSv1.3 Kx=any      Au=any   Enc=AESGCM(128)            Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256)            Mac=AEAD
(...)

Note:
The openssl ciphers command will output even ciphers that are not allowed, unless the -s switch is given. That option tells the command to list only supported ciphers.

All the options that can be set in the system_default_sect section are detailed in the SSL_CONF_cmd manpage.

Cipher strings, cipher suites, cipher lists

Encrypting data (or signing it) is not a one step process. The whole transformation applied to the source data (until it is in its encrypted form) has several stages, and each stage typically uses a different cryptographic algorithm. The combination of these algorithms is called a cipher suite.

Similar to GnuTLS, OpenSSL also uses the concept of cipher strings to group several algorithms and cipher suites together. The full list of cipher strings is shown in the openssl ciphers manpage.

OpenSSL distinguishes the ciphers used with TLSv1.3, and those used with TLSv1.2 and older. Specifically for the openssl ciphers command, we have:

  • -ciphersuites: used for the TLSv1.3 ciphersuites. So far, there are only five listed in the upstream documentation, and the defaults are:

    TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256

  • cipherlist: this is a plain argument in the command line of the openssl ciphers command, without a specific parameter, and is expected to be a list of cipher strings used in TLSv1.2 and lower. The default in Ubuntu Jammy 22.04 LTS is DEFAULT:@SECLEVEL=2.

These defaults are built-in in the library, and can be set in /etc/ssl/openssl.cnf via the corresponding configuration keys CipherString for TLSv1.2 and older, and CipherSuites for TLSv1.3. For example:

[system_default_sect]
CipherString = DEFAULT:@SECLEVEL=2
CipherSuites = TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256

In the end, without other constraints, the library will merge both lists into one set of supported crypto algorithms. If the crypto negotiation in a connection settles on TLSv1.3, then the list of CipherSuites is considered. If it’s TLSv1.2 or lower, then CipherString is used.

openssl ciphers examples

This will list all supported/enabled ciphers, with defaults taken from the library and /etc/ssl/openssl.cnf. Since no other options were given, this will include TLSv1.3 ciphersuites and TLSv1.2 and older cipher strings:

$ openssl ciphers -s -v
TLS_AES_256_GCM_SHA384         TLSv1.3 Kx=any      Au=any   Enc=AESGCM(256)            Mac=AEAD
TLS_CHACHA20_POLY1305_SHA256   TLSv1.3 Kx=any      Au=any   Enc=CHACHA20/POLY1305(256) Mac=AEAD
TLS_AES_128_GCM_SHA256         TLSv1.3 Kx=any      Au=any   Enc=AESGCM(128)            Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256)            Mac=AEAD
ECDHE-RSA-AES256-GCM-SHA384    TLSv1.2 Kx=ECDH     Au=RSA   Enc=AESGCM(256)            Mac=AEAD
DHE-RSA-AES256-GCM-SHA384      TLSv1.2 Kx=DH       Au=RSA   Enc=AESGCM(256)            Mac=AEAD
ECDHE-ECDSA-CHACHA20-POLY1305  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-RSA-CHACHA20-POLY1305    TLSv1.2 Kx=ECDH     Au=RSA   Enc=CHACHA20/POLY1305(256) Mac=AEAD
DHE-RSA-CHACHA20-POLY1305      TLSv1.2 Kx=DH       Au=RSA   Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-ECDSA-AES128-GCM-SHA256  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(128)            Mac=AEAD
ECDHE-RSA-AES128-GCM-SHA256    TLSv1.2 Kx=ECDH     Au=RSA   Enc=AESGCM(128)            Mac=AEAD
DHE-RSA-AES128-GCM-SHA256      TLSv1.2 Kx=DH       Au=RSA   Enc=AESGCM(128)            Mac=AEAD
ECDHE-ECDSA-AES256-SHA384      TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)               Mac=SHA384
ECDHE-RSA-AES256-SHA384        TLSv1.2 Kx=ECDH     Au=RSA   Enc=AES(256)               Mac=SHA384
DHE-RSA-AES256-SHA256          TLSv1.2 Kx=DH       Au=RSA   Enc=AES(256)               Mac=SHA256
ECDHE-ECDSA-AES128-SHA256      TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(128)               Mac=SHA256
ECDHE-RSA-AES128-SHA256        TLSv1.2 Kx=ECDH     Au=RSA   Enc=AES(128)               Mac=SHA256
DHE-RSA-AES128-SHA256          TLSv1.2 Kx=DH       Au=RSA   Enc=AES(128)               Mac=SHA256
ECDHE-ECDSA-AES256-SHA         TLSv1   Kx=ECDH     Au=ECDSA Enc=AES(256)               Mac=SHA1
ECDHE-RSA-AES256-SHA           TLSv1   Kx=ECDH     Au=RSA   Enc=AES(256)               Mac=SHA1
DHE-RSA-AES256-SHA             SSLv3   Kx=DH       Au=RSA   Enc=AES(256)               Mac=SHA1
ECDHE-ECDSA-AES128-SHA         TLSv1   Kx=ECDH     Au=ECDSA Enc=AES(128)               Mac=SHA1
ECDHE-RSA-AES128-SHA           TLSv1   Kx=ECDH     Au=RSA   Enc=AES(128)               Mac=SHA1
DHE-RSA-AES128-SHA             SSLv3   Kx=DH       Au=RSA   Enc=AES(128)               Mac=SHA1
AES256-GCM-SHA384              TLSv1.2 Kx=RSA      Au=RSA   Enc=AESGCM(256)            Mac=AEAD
AES128-GCM-SHA256              TLSv1.2 Kx=RSA      Au=RSA   Enc=AESGCM(128)            Mac=AEAD
AES256-SHA256                  TLSv1.2 Kx=RSA      Au=RSA   Enc=AES(256)               Mac=SHA256
AES128-SHA256                  TLSv1.2 Kx=RSA      Au=RSA   Enc=AES(128)               Mac=SHA256
AES256-SHA                     SSLv3   Kx=RSA      Au=RSA   Enc=AES(256)               Mac=SHA1
AES128-SHA                     SSLv3   Kx=RSA      Au=RSA   Enc=AES(128)               Mac=SHA1

Let’s filter this a bit, and just as an example, remove all AES128 ciphers and SHA1 hashes:

$ openssl ciphers -s -v 'DEFAULTS:-AES128:-SHA1'
TLS_AES_256_GCM_SHA384         TLSv1.3 Kx=any      Au=any   Enc=AESGCM(256)            Mac=AEAD
TLS_CHACHA20_POLY1305_SHA256   TLSv1.3 Kx=any      Au=any   Enc=CHACHA20/POLY1305(256) Mac=AEAD
TLS_AES_128_GCM_SHA256         TLSv1.3 Kx=any      Au=any   Enc=AESGCM(128)            Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256)            Mac=AEAD
ECDHE-RSA-AES256-GCM-SHA384    TLSv1.2 Kx=ECDH     Au=RSA   Enc=AESGCM(256)            Mac=AEAD
DHE-RSA-AES256-GCM-SHA384      TLSv1.2 Kx=DH       Au=RSA   Enc=AESGCM(256)            Mac=AEAD
ECDHE-ECDSA-CHACHA20-POLY1305  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-RSA-CHACHA20-POLY1305    TLSv1.2 Kx=ECDH     Au=RSA   Enc=CHACHA20/POLY1305(256) Mac=AEAD
DHE-RSA-CHACHA20-POLY1305      TLSv1.2 Kx=DH       Au=RSA   Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-ECDSA-AES256-SHA384      TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)               Mac=SHA384
ECDHE-RSA-AES256-SHA384        TLSv1.2 Kx=ECDH     Au=RSA   Enc=AES(256)               Mac=SHA384
DHE-RSA-AES256-SHA256          TLSv1.2 Kx=DH       Au=RSA   Enc=AES(256)               Mac=SHA256
AES256-GCM-SHA384              TLSv1.2 Kx=RSA      Au=RSA   Enc=AESGCM(256)            Mac=AEAD
AES256-SHA256                  TLSv1.2 Kx=RSA      Au=RSA   Enc=AES(256)               Mac=SHA256

Since we didn’t use -ciphersuites, the TLSv1.3 list was unaffected by our filtering, and still contains the AES128 cipher. But TLSv1.2 and older no longer have AES128 or SHA1. This type of filtering with ‘+’, ‘-’ and ‘!’ can be done with the TLSv1.2 and older protocols and is detailed in the openssl ciphers manpage.

To filter out TLSv1.3 algorithms, there is no such mechanism, and we must list explicitly what we want by using -ciphersuites:

$ openssl ciphers -s -v -ciphersuites TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256 'DEFAULTS:-AES128:-SHA1'
TLS_AES_256_GCM_SHA384         TLSv1.3 Kx=any      Au=any   Enc=AESGCM(256)            Mac=AEAD
TLS_CHACHA20_POLY1305_SHA256   TLSv1.3 Kx=any      Au=any   Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256)            Mac=AEAD
ECDHE-RSA-AES256-GCM-SHA384    TLSv1.2 Kx=ECDH     Au=RSA   Enc=AESGCM(256)            Mac=AEAD
DHE-RSA-AES256-GCM-SHA384      TLSv1.2 Kx=DH       Au=RSA   Enc=AESGCM(256)            Mac=AEAD
ECDHE-ECDSA-CHACHA20-POLY1305  TLSv1.2 Kx=ECDH     Au=ECDSA Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-RSA-CHACHA20-POLY1305    TLSv1.2 Kx=ECDH     Au=RSA   Enc=CHACHA20/POLY1305(256) Mac=AEAD
DHE-RSA-CHACHA20-POLY1305      TLSv1.2 Kx=DH       Au=RSA   Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-ECDSA-AES256-SHA384      TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)               Mac=SHA384
ECDHE-RSA-AES256-SHA384        TLSv1.2 Kx=ECDH     Au=RSA   Enc=AES(256)               Mac=SHA384
DHE-RSA-AES256-SHA256          TLSv1.2 Kx=DH       Au=RSA   Enc=AES(256)               Mac=SHA256
AES256-GCM-SHA384              TLSv1.2 Kx=RSA      Au=RSA   Enc=AESGCM(256)            Mac=AEAD
AES256-SHA256                  TLSv1.2 Kx=RSA      Au=RSA   Enc=AES(256)               Mac=SHA256

Config file examples

Let’s see some practical examples of how we can use the configuration file to tweak the default cryptographic settings of an application linked with OpenSSL.

Note that applications can still override these settings: what is set in the configuration file merely acts as a default that is used when nothing else in the application command line or its own config says otherwise.

Only use TLSv1.3

To configure the OpenSSL library to consider TLSv1.3 as the minimum acceptable protocol, we add a MinProtocol parameter to the /etc/ssl/openssl.cnf configuration file like this:

[system_default_sect]
CipherString = DEFAULT:@SECLEVEL=2
MinProtocol = TLSv1.3

If you then try to connect securely to a server that only offers, say TLSv1.2, the connection will fail:

$ curl https://j-server.lxd/stats
curl: (35) error:0A00042E:SSL routines::tlsv1 alert protocol version

$ wget https://j-server.lxd/stats
--2023-01-06 13:41:50--  https://j-server.lxd/stats
Resolving j-server.lxd (j-server.lxd)... 10.0.100.87
Connecting to j-server.lxd (j-server.lxd)|10.0.100.87|:443... connected.
OpenSSL: error:0A00042E:SSL routines::tlsv1 alert protocol version
Unable to establish SSL connection.

Use only AES256 with TLSv1.3

As an additional constraint, besides forcing TLSv1.3, let’s only allow AES256. This would do it for OpenSSL applications that do not override this elsewhere:

[system_default_sect]
CipherString = DEFAULT:@SECLEVEL=2
CipherSuites = TLS_AES_256_GCM_SHA384
MinProtocol = TLSv1.3

Since we are already forcing TLSv1.3, there is no need to tweak the CipherString list, since that applies only to TLSv1.2 and older.

The OpenSSL s_server command is very handy to test this (see the Troubleshooting section for details on how to use it):

$ sudo openssl s_server -cert j-server.pem -key j-server.key -port 443 -www

Note:
Be sure to use another system for this server, or else it will be subject to the same /etc/ssl/openssl.cnf constraints you are testing on the client, and this can lead to very confusing results.

As expected, a client will end up selecting TLSv1.3 and the TLS_AES_256_GCM_SHA384 cipher suite:

$ wget https://j-server.lxd/stats -O /dev/stdout -q | grep Cipher -w
New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384
    Cipher    : TLS_AES_256_GCM_SHA384

To be sure, we can tweak the server to only offer TLS_CHACHA20_POLY1305_SHA256 for example:

$ sudo openssl s_server -cert j-server.pem -key j-server.key -port 443 -www -ciphersuites TLS_CHACHA20_POLY1305_SHA256

And now the client will fail:

$ wget https://j-server.lxd/stats -O /dev/stdout
--2023-01-06 14:20:55--  https://j-server.lxd/stats
Resolving j-server.lxd (j-server.lxd)... 10.0.100.87
Connecting to j-server.lxd (j-server.lxd)|10.0.100.87|:443... connected.
OpenSSL: error:0A000410:SSL routines::sslv3 alert handshake failure
Unable to establish SSL connection.

Drop AES128 entirely

If we want to still allow TLS v1.2, but just drop AES128, then we need to configure the ciphers separately for TLS v1.3 and v1.2 or lower:

[system_default_sect]
CipherString = DEFAULT:@SECLEVEL=2:!AES128
CipherSuites = TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256
MinProtocol = TLSv1.2

To test, let’s force our test s_server server to only offer TLSv1.2:

$ sudo openssl s_server -cert j-server.pem -key j-server.key -port 443 -www -tls1_2

And our client picks AES256:

$ wget https://j-server.lxd/stats -O /dev/stdout -q | grep Cipher -w
New, TLSv1.2, Cipher is ECDHE-RSA-AES256-GCM-SHA384
    Cipher    : ECDHE-RSA-AES256-GCM-SHA384

But that could also be just because AES256 is stronger than AES128. Let’s not offer AES256 on the server, and also jump ahead and also remove CHACHA20, which would be the next one preferable to AES128:

$ sudo openssl s_server -cert j-server.pem -key j-server.key -port 443 -www -tls1_2 -cipher 'DEFAULT:!AES256:!CHACHA20'

Surely wget should fail now. Well, turns out it does select AES128:

$ wget https://j-server.lxd/stats -O /dev/stdout -q | grep Cipher -w
New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
    Cipher    : ECDHE-RSA-AES128-GCM-SHA256

It’s unclear why. Maybe it’s a safeguard, or maybe AES128 is always allowed in TLSv1.2 and we produced an invalid configuration. This case shows how crypto is complex, and also applications can override any such configuration setting that comes from the library. As a counter example, OpenSSL’s s_client tool follows the library config, and fails in this case:

$ echo | openssl s_client -connect j-server.lxd:443  | grep -w -i cipher
4007F4F9D47F0000:error:0A000410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:../ssl/record/rec_layer_s3.c:1584:SSL alert number 40
New, (NONE), Cipher is (NONE)

But we can override that as well with a command-line option and force s_client to allow AES128:

$ echo | openssl s_client -connect j-server.lxd:443 --cipher DEFAULT:AES128 2>&1| grep -w -i cipher
New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
    Cipher    : ECDHE-RSA-AES128-GCM-SHA256

References

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